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A144291 Triangular numbers n*(n-1)/2 with n and n -1 nonprime. 5
0, 36, 45, 105, 120, 210, 231, 300, 325, 351, 378, 528, 561, 595, 630, 741, 780, 990, 1035, 1176, 1225, 1275, 1326, 1485, 1540, 1596, 1653, 1953, 2016, 2080, 2145, 2346, 2415, 2775, 2850, 2926, 3003, 3240, 3321, 3570, 3655, 3741, 3828, 4095, 4186, 4278 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A000217(A068780(n-1)), n>1. - R. J. Mathar, Dec 10 2008

EXAMPLE

If n=1, then 1*(1-1)/2=0=a(1).

If n=9, then 9*(9-1)/2=36=a(2).

etc.

MAPLE

p:= -1: Res:= NULL: count:= 0:

while count < 100 do

  q:= p; p:= nextprime(p);

  if p - q > 2 then

    count:= count + p-q-2;

    Res:= Res, seq(k*(k+1)/2, k=q+1..p-2);

  fi

od:

Res; # Robert Israel, Jul 03 2018

MATHEMATICA

Reap[For[n = 1, n <= 100, n++, If[!PrimeQ[n] && !PrimeQ[n-1], Sow[n(n-1)/2] ] ] ][[2, 1]] (* Jean-Fran├žois Alcover, Mar 27 2019 *)

CROSSREFS

Cf. A000040, A000217, A068780, A141468.

Sequence in context: A168107 A048034 A195528 * A068143 A264961 A188630

Adjacent sequences:  A144288 A144289 A144290 * A144292 A144293 A144294

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, Dec 01 2008

EXTENSIONS

3570 inserted by R. J. Mathar, Dec 10 2008

STATUS

approved

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Last modified May 16 21:28 EDT 2021. Contains 343951 sequences. (Running on oeis4.)